MIC2166 Datasheet, PDF (15/26 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller

Micrel, Inc.
MIC2166
As described in the “Theory of Operation” subsection in
“Functional Description”, MIC2166 requires at least
20mV peak-to-peak ripple at the FB pin to make the gm
amplifier and the error comparator to behavior properly.
Also, the output voltage ripple should be in phase with
the inductor current. Therefore, the output voltage ripple
caused by the output capacitor C_OUT should be much
smaller than the ripple caused by the output capacitor
ESR. If low ESR capacitors, such as ceramic capacitors,
are selected as the output capacitors, a ripple injection
method should be applied to provide the enough FB
voltage ripples. Please refer to the “Ripple Injection”
subsection for more details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated below:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
(20)
The power dissipated in the output capacitor is:
P_{DISS(COUT )}
=
I_COUT
2
(RMS)
× ESR _COUT
(21)
Input Capacitor Selection
The input capacitor for the power stage input V_IN should
be selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to
high inrush currents, caused by turning on a “hot-
plugging”. A tantalum input capacitor’s voltage rating
should be at least two times the maximum input voltage
to maximize reliability. Aluminum electrolytic, OS-CON,
and multilayer polymer film capacitors can handle the
higher inrush currents without voltage de-rating. The
input voltage ripple will primarily depend upon the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
ΔV_IN = I_L(PK) × ESR_CIN
(22)
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
( ) I_CIN(RMS) ≈ I_OUT(max) × D × 1 − D
(23)
The power dissipated in the input capacitor is:
P_{DISS(CIN )} = I_CIN(RMS)2 × ESR_CIN
(24)
Voltage Setting Components
The MIC2166 requires two resistors to set the output
voltage, as shown in Figure 5.
Figure 5. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT
=
V_REF
× (1+
R1)
R2
(25)
where V_REF = 0.8V. If R1 is too large, it may allow noise
to be introduced into the voltage feedback loop. If R1 is
too small in value, it will decrease the efficiency of the
power supply, especially at light loads. The total voltage
divider resistance R1+R2 is recommended to be 7.5kΩ.
Once R1 is selected, R2 can be calculated using:
R2 = ^{VREF × R1}
(26)
V_OUT − V_REF
External Schottky Diode (Optional)
An external freewheeling diode can be used to keep the
inductor current flow continuous while both MOSFETs
are turned off.
The diode conducts current during the dead-time. The
dead-time prevents current from flowing unimpeded
through both MOSFETs and is typically 30ns. The diode
conducts twice during each switching cycle. Although the
average current through this diode is small, the diode
must be able to handle the peak current.
I_D(avg)CM = I_OUT × 2 × 30ns × f_SW
(27)
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_IN
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
(28)
where, VF = forward voltage at the peak diode current.
An external Schottky diode is recommended, even
though the low-side MOSFET contains a parasitic body
diode since the Schottky diode has much less forward
voltage than the body diode. The external diode will
improve efficiency and reduce the high frequency noise.
If the MOSFET body diode is used, it must be rated to
handle the peak and average current. The body diode
has a relatively slow reverse recovery time and a
relatively high forward voltage drop. The power lost in
the diode is proportional to the forward voltage drop of
the diode. As the high-side MOSFET starts to turn on,
June 2010
15
M9999-060810-B